VLT Diffraction‐limited Imaging and Spectroscopy in the NIR: Weighing the Black Hole in Centaurus A with NACO

We present high spatial resolution near-infrared spectra and images of thenucleus of Centaurus A (NGC 5128) obtained with NAOS-CONICA at the VLT. Theadaptive optics corrected data have a spatial resolution of 0.06" (FWHM) in K-and 0.11" in H-band, four times higher than previous studies. The observed gasmotions suggest a kinematically hot disk which is orbiting a central object andis oriented nearly perpendicular to the nuclear jet. We model the centralrotation and velocity dispersion curves of the [FeII] gas orbiting in thecombined potential of the stellar mass and the (dominant) black hole. Ourphysically most plausible model, a dynamically hot and geometrically thin gasdisk, yields a black hole mass of M_bh = (6.1 +0.6/-0.8) 10^7 M_sun. As thephysical state of the gas is not well understood, we also consider two limitingcases: first a cold disk model, which completely neglects the velocitydispersion; it yields an M_bh estimate that is almost two times lower. Theother extreme case is to model a spherical gas distribution in hydrostaticequilibrium through Jeans equation. Compared to the hot disk model the best-fitblack hole mass increases by a factor of 1.5. This wide mass range spanned bythe limiting cases shows how important the gas physics is even for highresolution data. Our overall best-fitting black hole mass is a factor of 2-4lower than previous measurements. With our revised M_bh estimate, Cen A'soffset from the M_bh-sigma relation is significantly reduced; it falls abovethis relation by a factor of ~2, which is close to the intrinsic scatter ofthis relation. (Abridged)Comment: 12 pages, 14 figures, including minor changes following the referee report; accepted for publication in The Astrophysical Journa